That same morning his forces appear to have dropped some ofthe most powerful conventional weapons yet used in the civilwar, in the rebel-held town of Raqqa. Evidence at the scene andwitness testimony led Human Rights Watch to conclude that the 14dead, many of them children, were killed by "vacuum bombs".

As his government works with U.N. inspectors to destroy itschemical weapons, the scale of Assad's remaining arsenal - andfaltering supplies to his enemies - suggest he need not feargiving up poison gas shells of the kind that killed hundreds inrebel areas two months ago and prompted threats from Washington.

Relative armed strength is hard to estimate and is only onefactor that may decide a war that has divided Syria on sectarianlines and drawn in rival foreign powers. But Assad's use of suchpowerful weaponry while international attention is on hischemical disarmament underlines the difficulties facing therebels - and their Western allies who want to force him out.

Air traffic data suggesting Qatar may have stopped shippingarms to Assad's opponents, and other evidence of supply problemsfor the rebels despite a U.S. pledge to help, may also helpexplain recent government gains. Western fears of Islamists inrebel ranks complicates efforts to arm other opposition groups.

"As worries grow over Islamist influence, the rebels seem tobe struggling more than they were to get supplies," said DavidHartwell, an analyst at IHS Jane's. "At the same time, thegovernment are throwing in everything they've got."

VACUUM BOMBS

Thermobaric or fuel-air explosives, known as vacuum bombs,are a small but fearsome part of the conventional array ofartillery, tanks and aircraft Syrian troops have deployed sincehostilities broke out in the wake of street protests in 2011.

Like much of Assad's equipment, experts believe the bombsthat hit Raqqa were Russian-made. Similar to devices in U.S.stocks, they detonate a cloud of vapour above the ground with amassive blast that sucks in oxygen from a wide area. That killspeople in a variety of ways, including by rupturing their lungs.

Used by Russian troops in Chechnya and Americans against theAfghan Taliban, critics say such bombs are too likely to killcivilians to be used in the mainly urban battlefields of Syria.

That has not deterred Assad's forces, which deny having usedchemical weapons, from using vacuum bombs on several occasions,according to opposition activists and independent observers whotrack online videos and other accounts of the fighting in Syria.

Recent evidence of Syrian forces dropping improvised "barrelbombs" and other makeshift explosives has led some analysts towonder if ordinary supplies are short. But one of the biggestarsenals in the region, intended to both fight Israel and localinsurgents, seems unlikely to be exhausted any time soon.

In the three years before fighting broke out, the U.S.Congressional Research Service estimates, Syria agreed more than$5 billion in arms purchases, most from Russia. Many expertsbelieve Russia and Iran have provided more since including spareparts to keep radar, jets and other systems running.

Syrian artillery batteries have sustained barrages on rebelareas lasting for days. In August, as well as the poison gasattacks near Damascus which the government blamed on rebels, theopposition said the Syrian army fired phosphorus and similaragents that burned the skin of civilians hit by it.

The military has also used vacuum bombs before, expertsmonitoring the conflict say. The attack at Raqqa most likelyinvolved Russian-made ODAB munitions, Human Rights Watch said.

The ODAB-500 PM was named last month by arms control groupAction On Armed Violence among "Syria's Dirty Dozen" - explosiveweapons too powerful or imprecise to be used in populated areas.

It also cited the T-72 tank, the world's biggest mortar theM240 and Grad rockets. Most such munitions have been used byAssad's forces, though rebels have captured and fired some - aswell as employing guerrilla tactics like suicide truck bombs.

REBEL PROBLEMS

Assad's troops appear on the back foot in some parts of thecountry, particularly the east. The Syrian Observatory for HumanRights says 47,000 soldiers and militia fighters loyal to Assadhave been killed since 2011, 40 percent of all the war's dead.